What is a dictionary in python and why do we need it?

In this article, we will discuss dictionaries in python and will try to answer questions like,

• What is a dictionary?
• Why do we need it?
• How to create a dictionary?
• How to access elements in a dictionary?

What is a dictionary in python?

In python, a dictionary is a kind of container that stores the items in key-value pairs, like,

This is an example of dictionary, which contain student names as keys and their age as values. Elements in the dictionary are stored as key-value pairs, where each value is mapped with a key. It is also known as associative array or hash table.

The above dictionary contains four items, i.e. four key-value pairs,

1. Jack & 32
2. Ritika & 31
3. Mark & 22
4. Mathew & 27

Why do we need dictionaries?

As a dictionary, keeps the elements in key-value mapping format and internally uses hashing for it; therefore, we can get a value from the dictionary by its key very quickly. In best cases, its complexity is O(1), whereas, in the worst case, its complexity can be O(n).

If you want to know more about hashing check this article –> What is Hashing and Hash Table?

How to create a dictionary?

To create a dictionary, we can use curly braces, i.e., {}.

dictObj = {}

It will create an empty dictionary.

Now to create a dictionary with items, pass the key-value pairs in curly braces  {}. Also, in every couple, key & value will be separated by a colon (:).

Let’s understand by some examples,

Create a dictionary that contains student names as keys and their age as value i.e.

student_age = {'Jack': 32, 'Ritika': 31, 'Mark' : 22, 'Mathew' : 27}

This dictionary contains four items. We can print the dictionary to check it contents,

print(student_age)

Output:

{'Jack': 32, 'Ritika': 31, 'Mark': 22, 'Mathew': 27}

Essential points about keys in the dictionary

• Keys are always unique in the dictionary
• keys must be of an immutable data type, i.e., strings, numbers, or tuples.
  • It means once a key-value pair is added in the dictionary then it cannot modify the key itself, although we can change the value associated with it.

For example, if we create a dictionary with duplicate keys and print its content,

student_age = {'Jack': 32, 'Ritika': 31, 'Jack' : 22}

print(student_age)

Output:

{'Jack': 22, 'Ritika': 31}

The value of jack is overwritten because keys are always unique in a dictionary.

Important points about value in dictionaries.

The values in a dictionary can be of any type.

For example, let’s create a dictionary where key is integer and value is a list of strings i.e.

student_info = {11: ['varun', 'Delhi', 9911],
               12: ['Jack', 'London', 2211],
               13: ['Ritika', 'Mumbai', 3311]}

Similarly, a dictionary can have any object as the value.

How to access an item in the dictionary

Suppose we have a dictionary like this,

student_age = {'Jack': 32, 'Ritika': 31, 'Mark' : 22, 'Mathew' : 27}

We can access a specific item/pair in a dictionary using [] operator on the dictionary object. If we call the operator [] on the dictionary object and pass a key, then it will return its value is like this,

age = student_age ['Jack']

print('Age of Jack is : ', age)

Output:

Age of Jack is :  32

Now if we pass a key that doesn’t exist in the dictionary, then it returns a KeyError,

age = student_age ['abc']

Error

KeyError: 'abc'

So, we before accessing any value in a dictionary, we should first check if given keys exist in the dictionary or not i.e.

if 'abc' in student_age:
    age = student_age['abc']

Python Dictionary Tutorial - Series:

1. What is a Dictionary in Python & why do we need it?
2. Creating Dictionaries in Python
3. Iterating over dictionaries
4. Check if a key exists in dictionary
5. Check if a value exists in dictionary
6. Get all the keys in Dictionary
7. Get all the Values in a Dictionary
8. Remove a key from Dictionary
9. Add key/value pairs in Dictionary
10. Find keys by value in Dictionary
11. Filter a dictionary by conditions
12. Print dictionary line by line
13. Convert a list to dictionary
14. Sort a Dictionary by key
15. Sort a dictionary by value in descending or ascending order
16. Dictionary: Shallow vs Deep Copy
17. Remove keys while Iterating
18. Get all keys with maximum value
19. Merge two or more dictionaries in python

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The complete example is as follows,

def main():

    # Creating an empty dictionary
    dictObj = {}

    print('** Create dictionary of student names & their age ***')
    student_age = {'Jack': 32, 'Ritika': 31, 'Mark' : 22, 'Mathew' : 27}

    print(student_age)


    print('*** Dictionary with duplicate keys ***')

    student_age = {'Jack': 32, 'Ritika': 31, 'Jack' : 22}

    print(student_age)

    student_info = {11: ['varun', 'Delhi', 9911],
                   12: ['Jack', 'London', 2211],
                   13: ['Ritika', 'Mumbai', 3311]}


    print(student_info)
    
    print('*** Accessing items in a dictionary ***')

    student_age = {'Jack': 32, 'Ritika': 31, 'Mark': 22, 'Mathew': 27}

    # Access the value of item with key jack
    age = student_age['Jack']

    print('Age of Jack is : ', age)

    # Passing a key in [] that don't exist,
    if 'abc' in student_age:
        age = student_age['abc']


if __name__ == '__main__':
  main()

Output:

** Create dictionary of student names & their age ***
{'Jack': 32, 'Ritika': 31, 'Mark': 22, 'Mathew': 27}
*** Dictionary with duplicate keys ***
{'Jack': 22, 'Ritika': 31}
{11: ['varun', 'Delhi', 9911], 12: ['Jack', 'London', 2211], 13: ['Ritika', 'Mumbai', 3311]}
*** Accessing items in a dictionary ***
Age of Jack is :  32